The present invention relates generally to valve actuators. More specifically, the present invention relates to override and backup systems for subsea valve actuators. Still more specifically, the present invention relates to override systems for subsea valve applications.
Increasing performance demands for subsea hydrocarbon production systems have led to a demand for high performance control systems to operate subsea pressure control equipment, such as valves and chokes. Hydraulic actuators are used to operate many of the pressure control equipment used subsea. Pressurized hydraulic fluid may be supplied to the hydraulic actuators by a direct hydraulic control system or an electrohydraulic control system. Direct hydraulic control systems provides pressurized hydraulic fluid directly from the surface to the subsea valve actuators. Electrohydraulic control systems utilize electrical signals transmitted to an electrically actuated valve manifold that controls the flow of hydraulic fluid to the hydraulic actuators of the pressure control equipment.
The performance of both direct hydraulic and electrohydraulic control systems is affected by a number of factors, including the water depth in which the components operate, the distance from the platform controlling the operation, and a variety of other constraints. Thus, as water depth and field size increases, the limits of hydraulic control systems become an increasing issue. Further, even when the use of a hydraulic control system is technically feasible, the cost of the system may preclude its use in a smaller or marginal field.
In order to provide an alternative to hydraulic control systems, full electrical control systems, including electric actuators, have been developed. Instead of relying on pressurized hydraulic fluid to actuate the pressure control components, electrical actuators are supplied with an electric current. The reliance on electric current can allow for improved response times, especially over long distances and in deep water.
Even with electrical control systems and actuators, many operators still desire some sort of system that allows for operation of the actuators in the case of failure of the electric actuator or interruption in the supply of electrical current. In certain applications, an operator may want to be able to override the electrical control system and operate a valve, or some other components, via remote operation or direct intervention, such as with a remotely operated vehicle (ROV).
Thus, there remains a need to develop methods and apparatus for allowing operation of subsea actuators that overcome some of the foregoing difficulties while providing more advantageous overall results.